This invention relates to a measuring circuit of a double beam or 2-wave-length system for photometers, which utilize the light from a single light source, and more particularly to a device for correcting the sensitivities of two light detectors, which are used to detect the light passing through an object sample and the light passing through a reference sample, in such a manner that the sensitivities of these detectors become equal to each other.
In order to determine the absorbance of an object sample by a photometer, a device for applying the light to an object sample and a reference sample, detecting the light passing through these samples, by a plurality of detecting means, i.e. a plurality of light detectors, and determining a ratio of levels of signals detected thereby is widely used. What is important for such a photometer is to set the sensitivities of a light detector for an object sample and a light detector for a reference sample to the same level. When the sensitivities of these light detectors are different, the measurement values of transmissivities and absorbances of samples become different; measurement errors occur. Accordingly, when a plurality of light detectors are used together, it is necessary that the light detectors have the closest possible characteristics. However, it is impossible to obtain light detectors having completely the same temperature characteristics and completely the same wave-length and sensitivity characteristics. Therefore, when an ambient temperature varies, or when a measuring wave-length is changed by passing the light through spectroscopes or filters, measurement errors occur.
In a conventional photometer, the light from a single light source is subjected to a time division system to alternately detect the light passing through an object sample and the light passing through a reference sample, and thereby eliminate the drawbacks encountered in the above-described photometer utilizing a plurality of light detectors. However, such a conventional photometer using a time division system also has problems peculiar thereto. Namely, in a time division system, the time, during which signals can be obtained, is reduced to more than half, and only such signals that correspond to different instants can be obtained. This tends to cause a decrease in the measuring accuracy. When signals include noise having a frequency close to a time division frequency, beat occurs. When signals include noise having a frequency higher than a time division frequency, the responding capability thereof lowers, and it becomes difficult to eliminate a noise component. A conventional photometer employing a time division system has many drawbacks mentioned above. In a time division system, a chopper is used. Since a rotational speed of a chopper is limited, a phenomenon varying at a high speed cannot be measured faithfully. Especially, in an atomic absorption analyzer lately developed, the high-speed atomization is carried out, and a time division system does not actually serve the purpose.